Described below is a coil device having a coil winding formed of a superconducting tape conductor.
In the field of superconducting machines and superconducting magnet coils, coil devices in which superconducting wires or tape conductors are wound in coil windings are known. For classical low-temperature superconductors such as NbTi and Nb3Sn, conductors in wire form are conventionally used. High-temperature superconductors or high-Ta superconductors (HTS), however, are superconducting materials with a critical temperature above 25 K, and for some material classes above 77 K. These HTS conductors are typically in the form of flat tape conductors formed of a strip-shaped substrate tape and a superconducting layer arranged on the substrate tape. In addition, the tape conductors often also have further layers such as stabilization layers, buffer layers, and in many cases also insulation layers.
The most important material class of so-called second-generation HTS conductors (2G HTS) is compounds of the type REBa2Cu3Ox, where RE stands for a rare earth element or a mixture of such elements. Many superconducting tape conductors formed of such ceramic superconducting layers are very sensitive to mechanical loads and must therefore be protected from mechanical loads, such as tensile, compressive or shear stresses, both during production and during operation of the superconducting coils.
When electrical coils are produced from superconducting tape conductors, either successive windings of the tape conductors are typically already adhesively bonded to one another by an impregnating resin during winding or the finished wound coil is subsequently encapsulated with an encapsulation medium. Typical encapsulation media in this case are epoxy resins, with which the coil may for example be encapsulated by a vacuum encapsulation method. The effect of the adhesive bonding or the encapsulation of the coil windings is that the finished coil is protected from mechanical loads, for example due to Lorentz forces in strong magnetic fields and/or due to centrifugal forces in the case of rapid rotation.
One problem with the use of superconducting coils is the different thermal contraction of the various materials in the coils when cooling to operating temperature. During cooling to an operating temperature of for example from 30 K to 70 K, above all the polymer constituents of the adhesive and/or of the encapsulation compound, as well as insulator materials possibly present, are subject to greater thermal shrinkage than the metallic and ceramic constituents of the tape conductor. The different thermal contraction leads during and after cooling to the formation of stresses, which may cause damage of the superconducting layer. The use of a winding carrier having thermal contraction greater than that of the tape conductor may also cause the formation of radial tensile stresses perpendicular to the plane of the tape conductor, and therefore compression of the superconducting layer. Above all, radial tensile stresses lead much more easily than possible radial compressive stresses to damage of the superconducting properties, possibly to the extent of delamination of the superconducting layer from the substrate of the tape conductor. A radial tension causes inner-lying layers of the coil winding to be pulled in the direction of the inside of the coil, and therefore causes the tape conductor to be compressed in the longitudinal direction. The damage due to this can lead to a reduction of up to 60% in the maximum operating current, which makes the conventional winding methods for superconducting coils incompatible with modern 2G HTS materials.
The application with the official file reference 102011077457.2, not yet published at the priority date of the present application, describes a coil superconducting winding in which a superconducting tape conductor is wound on a winding carrier in such a way that there is a positive radial pressure between the layers of the coil winding both at room temperature and at an operating temperature of the coil. This can be achieved by suitable selection of the winding carrier and of the winding tension, as well as by a weakly configured connection of the winding and the winding carrier. Nevertheless, even with a correspondingly produced coil in which the winding carrier does not contribute to the formation of tensile stresses, unfavorable tensile stresses can occur merely because of the differences in the thermal contractions of the various materials in the winding. Particularly in the case of large windings having more than for example 100 turns, large tensile stresses which greatly impair the superconducting properties of the coil can occur because of this effect.